This project examines the immunological mechanisms which involved in the pathogenesis of autoimmune- and infectious diseases of the central nervous system such as multiple sclerosis (MS) and chronic Lyme disease. We characterize the fine specificity, function and phenotype of T lymphocytes in the above diseases. These experiments allow a better understanding of the foreign antigens that may trigger autoimmune responses in MS and also of their function with respect to cytokine secretion and chemokine receptor expression. Based on this knowledge the project attempts to develop both specific immunomodulatory treatments such as altered peptide ligands (APL) or therapies that influence immune recognition in MS in a broader way. Examples of the latter are the humanized antibody against the interleukin-2 receptor alpha chain (daclizumab). Treatment of relapsing-remitting MS patients failing interferon-beta with daclizumab has been well tolerated. The initial clinical trial that involved the treatment of patients already on interferon beta resulted in a marked decrease in contrast enhancing lesion (78%) as well as an improvement in clinical findings although the study was very small; 10 patients with relapsing remitting disease. In addition to the evidence of efficacy, the CIS has carefully studied the mechanisms of action of this treatment. The results have lead to the surprising finding that daclizumab acts primarily via expansion of immunoregulatory NK cells. The findings provide evidence for a unique immunorulatory pathway. These studies will help us to understand better the complex mechanism of action of these compounds and eventually also the disease pathogenesis itself. All the clinical projects are being pursued in close collaboration with the Neurological Disease Section Section/Office of the Chief. A second example of using immunological tools to examine the mechanisms of action of a potential therapy in MS include studies of hematopoietic stem cell transplantation (HSCT). HSCT represent an aggressive form of therapy involving severe immunosuppression followed by rescue with hematopoietic, CD34+, stem cells. The therapy has been studied in several small studies of MS patients as well as studies of patients with other autoimmune disorders. However debate exists not only over the effectiveness but also the mechanism. With respect to the latter the question is if this simply represents a form of aggressive immunosuppression or if it leads to a reprogramming of the immune repertoire. In a cohort of patients followed for two years post transplantation an expansion of naive T cells was demonstrated by TREC analysis. Analysis of the repertoire indicated that the diversity was increased suggesting a reprogramming of the repertoire. These studies are now being extended into a larger cohort with well defined clinical outcome measures. Another important project, which is currently being pursued at NIB, NINDS, NIH, addresses the question which foreign antigens, e.g. viruses or bacteria, may trigger the initiation or exacerbations of disease via a mechanism referred to as molecular mimicry. This concept refers to cross-recognition between autoantigens, e.g. derived from the myelin sheath, and antigens derived from foreign agents. For this purpose, we currently employ a novel methodology called combinatorial peptide libraries in the positional scanning format (ps-SCL) together with bioinformatic approaches to identify the entire spectrum of stimulatory ligands for autoreactive T cell clones derived from MS patients. In brief, we test T cell clones with ps-SCL, which represent highly complex mixtures of trillions of peptides, and deduce stimulatory peptide sequences from these assays before we screen the databases of all known protein sequences for potential stimulatory peptides.